This invention relates to automatic rangefinders of the type generally described in U.S. Pat. No. 4,002,899 assigned to Honeywell, Inc. Such automatic rangefinders and focusing systems include two optical system paths that bring light from the subject to an optical detector. Such optical detectors are photosensitive devices and as such respond to the fact that both optical systems are aligned with one another and are directed toward the same subject. This alignment is called coincidence and generates a pulse from the optical detector which can be used to adjust the focus of the main camera lens.
In using such automatic rangefinding optical devices with a main lens which has the capacity for zooming, difficulties arise in that a plurality of subjects within the depth of field tend to confuse the automatic focusing by introduction of simultaneous signals particularly during zooming of the main lens. In the instance where two subjects are in the field of view and one subject is closer than the other the zoom lens in its telephoto position will be out of focus because the automatic rangefinding devices tend to detect both subjects and cannot rapidly accommodate a zooming change. It is an object of this invention to overcome the aforesaid problem and to detect the subject and focus precisely by varying the objective images which are brought along the optical system paths to the automatic rangefinder in accordance with the variation of the main objective image brought along the optical axis through the zoom lens to the image plane of the camera.